Austenitic Stainless Steel for Cold Working Suitable For Later Machining

ABSTRACT

Austenitic stainless steel for cold working suitable for later machining, characterized by the following composition by weight:
         carbon&lt;0.030%   0.3%&lt;silicon&lt;1%   7%&lt;manganese&lt;9%   4.55%&lt;nickel&lt;7%   molybdenum&lt;0.8%   2%&lt;copper&lt;4%   0.02%&lt;nitrogen&lt;0.060%   sulfur&lt;0.01%   phosphorus&lt;0.030%

This is a Continuation-In-Part of application Ser. No. 10/201,968 filedJul. 25, 2002. The entire disclosure of the prior applications is herebyincorporated by reference.

The invention concerns an austenitic stainless steel for cold workingsuch as cold heading. Steels containing a minimum of 11% chrome arecalled stainless and exhibit good corrosion resistance. Austeniticstainless steels also contain nickel, which provides valuable propertiesfor processing, particularly cold working

BACKGROUND OF THE INVENTION

One of the most commonly used austenitic steels has the followingcomposition: C<0.08, Si<1%, Mn<2%, S<03%, P<0.045%, 18<Cr 20%, 8<Ni<12%.This steel is well suited for cold working, but has a limited formingrange due to the relatively high hardening coefficient which leads tosignificant work hardening via the formation of hardening martensite.

Another austenitic stainless steel is known with the followingcomposition: C<0.12%; Si<1%; Mn<2%; S<0.03%; P<0.045%; 17<Cr<19%;10<Ni<13%, providing a solution to the hardening issue through arelatively high nickel content, which limits the formation of hardeningmartensite. However, the presence of nickel in such concentrations alsolimits the elongation properties of the steel; furthermore, this steelis costly due to the high nickel content.

An interesting alternative has been partially identified by addingcopper to the composition. Indeed, copper acts in the same way asnickel, limiting the formation of hardening martensite. There is,however, an upper limit for copper concentration, typically 3.5%, whichshould not be exceeded in order to prevent burning while hot rolling thesteel. Nevertheless, such high copper content provides good formingcapabilities, enabling the overall nickel content to be diminished toaround 8%. Such a steel structure is particularly employed in the coldheading of long stainless steel products.

A widespread approach that improves to some extent the cold processingof austenitic stainless steels is to add manganese to the composition,which acts in the same way as nickel or copper. Several patents proposesolutions of this nature. For instance,

-   -   French patent 2,229,776 claims a steel with the following        composition: 3<Ni<15%; 6%<Mn<16%; 10%<Cr<25%; Si>2%.        This steel has valuable abrasion resistance properties.    -   U.S. Pat. No. 3,910,788 describes a steel with the following        composition: 1%<Si<2.5%; 1.5%<Mn<5%; 1%<Cu<4%; 6%<Ni<9%;        15%<Cr<19%; N<0.03%; C+N<0.04%.

This steel is suitable for cold working and die-stamping in the flatproduct industry and is resilient to delayed failure. The nickel contentin the composition is relatively high; the steel also contains sensibleamounts of silicon to increase the hardening coefficient.

-   -   Japanese patents J63060051 and J63060050 both propose the        following steel composition: C<0.15%; Si<1.5%; 0.5%<Mn<6%;        17%<Cr<23%; 10%<Ni<15%; 0.1%<Cu<3%; 0.02%<N<0.35%; with        stabilization using chemical elements Ti+Nb+V. The high carbon        and nitrogen contents result in high hardness being rapidly        attained during cold working, which disqualifies the steel for        cold heading applications.    -   U.S. Pat. No. 3,753,693 presents a steel composition such as:        17<Cr<19%; 7%<Ni<10%; 11%<Mn<13%; 0.01%<N<0.07%; C<0.06%; Si<1%;        Mo<2%; Cu<1.5%.

This composition is particularly well-suited for cold headingapplications where it provides a low hardening coefficient due to thevery high nickel and manganese contents.

While being well-suited for cold heading, this composition has noeconomical interest because of the high nickel and chrome contents.

-   -   JP 55,031,173 presents a grade with the following composition:        C<0.02%; 0.04%<N<0.1%; 2.5%<Cu<4%; 6%<Ni<8%; 17%<Cr<19% and        3%<Mn<4%; S<0.003%.

The document provides a relatively high nickel content, and a very highnitrogen content. This steel is used to manufacture stainless steelrivets and screws.

-   -   U.S. Pat. No. 4,911,883 concerns a stainless steel for cold        working, with the following composition: C<0.04%; Si<0.6%;        6%<Ni<8%; 2.2%<Mn<3.8%; 17%<Cr<19%; 2.5%<Cu<4%; S<0.002%;        N<0.010%.

This composition is similar to that previously cited, but has a ratherhigh nickel content.

-   -   WO 00/26428 describes the following composition: 0.025%<C<0.15%;        4%<Mn<12%; Si<1%; P<0.2%; S<0.1%; 15.5%<Cr<17.5%; 1%<Ni<4%;        0.25%<Mo<1.5%; 1.5%<Cu<4%; W<1%; Co<1%; 0.05%<N<0.3%; with the        conditions Cr %+Mo %<17.75% and Cr %+3.3Mo %+13N %>20.5%. This        composition has a low nickel content.        In the same category, JP 2001011579A presents the following        composition: 0.05%<C<0.5%; 6%<Mn<15%; Si<0.5%; S<0.03%;        10%<Cr<20%; 0.04%<Ni<0.3%; Mo<3%; Cu<3%; Al<0.1%; 0.05%<N<0.3%,        with a relatively high nitrogen+carbon content, as in the        composition proposed by the previous document.

In conclusion, solutions proposed to obtain compositions of austeniticstainless steel for cold working containing manganese may be classifiedin the following manner:

-   -   highly alloyed grades, for very specific applications;    -   nitrogen and/or carbon grades with high mechanical        characteristics;    -   less alloyed grades, with relatively high levels of nickel, and        therefore relatively low levels of manganese, providing little        improvement over classical grades, both in terms of workability        and cost reduction.

SUMMARY OF THE INVENTION

The goal of the invention is to propose an austenitic stainless steelfor cold forming suitable for later machining exhibiting improvedmechanical properties over standard austenitic stainless steels; theseproperties are very valuable for cold working applications, particularlycold heading.

The goal of the invention is an austenitic stainless steel for coldworking suitable for later machining, characterized by the followingcomposition by weight:

-   -   carbon<0.030%    -   0.3%<silicon<1%    -   7%<manganese<9%    -   4.55%<nickel<7%    -   15%<chromium<18%    -   molybdenum<0.8%    -   2%<copper<4%    -   0.02%<nitrogen<0.060%    -   sulfur<0.01%    -   phosphorus<0.030%        The other characteristics of the invention are:    -   the composition also comprises boron in a concentration below        50·10⁻⁴%;    -   the Md30 index, defined by the formula: Md30=551−462(C %+N        %)−9.2Si %−20Mn %−13.7Cr %−29Ni %−29Cu %−18.5Mo %, is below −60.    -   the ferrite rate after reheating of the raw solidification        structure to 1,240° C. is below 10%, as per the following        formula:        % Ferrite=0.034 x²+0.284x−0.347, where        x=6.903[Cr_(eq)−Ni_(eq)/1.029−6.998], with Cr_(eq)=Cr % and        Ni_(eq)=Ni %+20.04 C %+21.31 N %+0.46 Cu %+0.08 Mn %.

DESCRIPTION OF THE INVENTION

The following description, provided without limitation, will facilitatethe understanding of the invention. The invention concerns an austeniticstainless steel for cold working suitable for later machining,characterized by the following composition by weight:

-   -   carbon<0.030%    -   0.3%<silicon<1%    -   7%<manganese<9%    -   4.55%<nickel<7%    -   15%<chromium<18%    -   molybdenum<0.8%    -   2%<copper<4%    -   0.02%<nitrogen<0.060%    -   sulfur<0.01%    -   phosphorus<0.030%

In the proposed composition, the carbon content is controlled to bebelow 0.030% in order to limit the formation and hardening of hardeningmartensite. In the same manner, and for the same purposes, the combinedcarbon and nitrogen content must be controlled to be below 0.07%.

The nitrogen content is controlled to a level between 0.02%<N<0.060%,preferably 0.02%<N<0.04%, in order to stabilize the austenite andguarantee a hot ferrite content <15%. Indeed, to facilitate hot working,the ferrite rate must be maintained below 10% after reheating of the rawsolidification structure to 1,240° C., as per the following formula: %Ferrite=0.034 x²+0.284x−0.347, wherex=6.903[Cr_(eq)−Ni_(eq)/1.029−6.998], with Cr_(eq)=Cr % and Ni_(eq)=Ni%+20.04 C %+21.31 N %+0.46 Cu %+0.08 Mn %.

In the composition as per the invention, the nickel content has beenlowered to between 4.55% and 7%, preferably to around 5%.

The lower nickel content is compensated by a manganese content between7% and 9%, preferably around 8%. More specifically, with thesubstitution of nickel by manganese in austenitic stainless steels,manganese levels around 6% should be avoided as deleterious ferrite maybe produced during industrial processing or subsequent heat treatmentsuch as solution annealing.

As an example, stainless steels containing 5% Ni with various amounts ofmanganese have been processed as follow : (1) hot deformation of castproducts and (2) reheating for 15 min at 1080° C. followed by fastcooling to room temperature. Ferrite measurement was performed bymetallographic examination of polished samples.

Steel A B C D % C 0.023 0.026 0.023 0.027 % Mn 4.0 5.0 6.0 7.0 % Si 0.300.30 0.30 0.31 % S 0.004 0.004 0.004 0.004 % P 0.008 0.008 0.008 0.008 %Ni 4.95 4.99 4.99 4.97 % Cr 17.0 17.1 17.1 17.1 % Mo <0.005 <0.005<0.005 <0.005 % Cu 3.0 3.1 3.1 3.1 % N 0.027 0.027 0.028 0.028 % S 0.0040.004 0.004 0.004 % P 0.008 0.008 0.008 0.008 Ferrite 1.5 2.7 4.7 2.9Content Calc. Md30 −18 −45 −64 −86

In the above, the highest ferrite content heat exhibits the lowest coldheading ability.

Moreover, another drawback with high ferrite content is the potential ofcorrosion healing due to carbide precipitation at the ferrite/austeniteboundaries. The balance in austenite forming and ferrite forming agentsimply that when Ni is set to as low as 5%, the content in manganeseshould be increased to 7% or above.

The presence of chrome guarantees good corrosion resistance, with thecontent chosen between 15% and 18%.

Copper is present in the composition to stabilize the austenite. Asdescribed in prior art, it is limited to below 4%. A content above 2% ispreferred to improve stabilization of the austenite.

Sulfur is limited to a content below 0.01% and preferably below 0.002%in order to limit the formation of manganese sulfides which aredetrimental to hot workability and corrosion resistance.

Other elements are also controlled, e.g. silicon content should be below1%, preferably below 0.3%. The composition also contains molybdenum in aconcentration below 0.8% and phosphorus in a concentration below 0.030%.

Boron may be added in a concentration below 50.10⁻⁴% to facilitate coldand hot working.

Eventually, the Md30 index of the steel must be as low as possible,preferably below −60; the value of Md30 is given by the followingformula: Md30=551−462(C %+N %)−9.2Si %−20Mn %−13.7Cr %−29Ni %−29Cu%−18.5Mo %.

In an example of implementation, the composition of a steel 1 as per theinvention was compared to that of a reference sample, a standardaustenitic steel with the composition presented in table 1 below:

TABLE 1 % C % Si % Mn % Ni % Cr % Mo % Cu % N % S Md30 %* Steel 1 0.0230.36 8.1 5.1 15.0 0.30 3.3 0.035 0.001 −71.6 Ref. steel 0.029 0.33 1.88.1 17.2 0.29 3.2 0.027 0.002 −59.3

For cold heading applications, the steel as per the invention exhibitsimproved cold workability compared to the standard reference steel.Cruciform impact testing shows that:

-   -   Steel 1 as per the proposed invention does not exhibit any        cracking;    -   The reference steel exhibits incipient ductile failure at the        angles of the cross.

For corrosion, the steel as per the invention exhibits good corrosionresistance, comparable to that of a standard 304 steel.

For magnetism, welding and machining, the steel as per the invention isnon-magnetic in annealed temper, and lightly magnetic in hard temper;has a weldability equivalent to that of a standard 304 steel; and amachinability close to that of a standard 304 steel.

The steel as per the invention is commercially attractive seeing as itemploys alloy elements which are less costly than nickel, andfacilitates working at each stage of processing, particularly due to ahot structure and mechanical properties which are compatible withexisting processing techniques for standard austenitic stainless steels.

The steel as per the invention is well-suited for parts involving aninitial cold heading operation followed by a finishing machiningoperation.

Its field of application may include fitting products such as fasteners,bolting, nuts, threaded rods, and any particular parts manufactured bycold heading, e.g. fixtures, automobile parts such as mountings,airbags, probe support and body elements, or mass-produced connectionparts such as electrical connection equipment.

Other applications can also be cited that significantly involve coldworking in order to provide a good performance/cost ratio, e.g. in thefields of drawn wires and fine wires, filters and screens, or springmanufacturing.

1. Austenitic stainless steel for cold working suitable for latermachining, characterized by the following composition by weight:carbon<0.030% 0.3%<silicon<1% 7%<manganese<9% 4.55%<nickel<7%15%<chromium<18% molybdenum<0.8% 2%<copper<4% 0.02%<nitrogen<0.060%sulfur<0.01% phosphorus<0.030%
 2. Steel as per claim 1, characterized inthat the composition also comprises boron in a concentration below50.10⁻⁴%.
 3. Steel as per claim 1, characterized in that the Md30 index,defined by the formula: Md30=551−462(C %+N %)−9.2Si %−20Mn %−13.7Cr%−29Ni %−29Cu %−18.5Mo %, is below −60.
 4. Steel as per claim 1,characterized in that the ferrite rate after reheating of the rawsolidification structure to 1,240° C. is below 10%, as per the followingformula: % Ferrite =0.034 x²+0.284x−0.347, where x=6.903[Cr_(eq)−Ni_(eq)/1.029−6.998], with Cr_(eq)=Cr % and Ni_(eq)=Ni %+20.04C %+21.31 N %+0.46 Cu %+0.08 Mn %.